A displaying gray scale of pixels of a display product may be adjusted by setting a gamma voltage. In a gamma voltage debugging method for an electroluminescent display device in the related art, a luminance meter has to be employed to monitor the luminance to identify the proper gamma voltage during the process of changing the voltage. Since the luminance and the voltage value should be measured and recorded at the same time, there may be an error due to decay of the luminance. Moreover, since many parameters need to be measured and the apparatus is complex, the gamma voltage debugging process is long and not suitable for mass production.